Optical disk substrate, optical information-storage medium, and process and apparatus for manufacturing the optical disk substrate

ABSTRACT

An optical disk substrate having a foreign-substances index of not more than 1×10 5  μm 2  /g, an optical information-storage medium wherein the substrate has an information-recording layer, and a propocess and an apparatus for manufacturing the optical disk substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk having a reduced amountof foreign substance, an optical information-storage medium, and aninjection molding process and apparatus suitable for manufacturing theoptical disk substrate.

2. Description of the Related Art

An optical information-storage medium, such as an audio-disk, a laserdisk, an optical disk memory, and a magneto-optical disk, records and/orreads out information by utilizing laser beams. In the optical storagemedium, a transparent substrate, i.e., an optical disk substrate,prepared from polycarbonate, polymethyl methacryrate, glass or the like,is used, and foreign substances, such as dust or carbonized material,contained in the optical disk substrate have a considerable affect onthe reliability of a recording and/or a reading out of information.

In consideration of the above circumstances, the foreign substancescontained in a raw material were filtered off, during a purification orgranulation step, to reduce the amount thereof. For example, JapaneseUnexamined Pat. Publication (KOKAI) No. 61-90345 and No. 63-91231disclose such a filtration method.

The above Japanese Unexamined Pat. Publication (KOKAI) No. 61-90345discloses that an amount of the foreign substances having a particlediameter of 0.5 μm or more should be 1 ×10⁵ /g or less in the opticaldisk substrate, and to comply with the above requirement, the foreignsubstances contained in the raw material such as a monomer should beremoved by distillation and/or filtration, the manufacturing apparatusmust be kept clean, and further, an inclusion of the foreign substancesduring a manufacturing step should be prevented.

The above Japanese Unexamined Pat. Publication (KOKAI) No. 63-91231discloses that an amount of foreign substances having a particlediameter of 1 μm or more should be 10000/g or less in the storagemedium, and to comply with the above requirement, the foreign substancesare removed by filtration of a solution in an organic solvent, andfurther, the storage medium is formed from a resin composition which hasbeen passed through a sintered metallic filter, in a molten state, toremove the fine foreign substances.

Although the above patent publications mention that the number offoreign substances having a particle diameter of at least 0.5 μm or 1 μmper 1 g of the raw material affects the recording and/or reading-outcharacteristics, for example, an error ratio (i.e., bit error ratio:BER), this number is not always an appropriate indicator for anevaluation thereof.

The amount of foreign substances cannot be sufficiently reduced only byremoving the foreign substances from the unmelted resin, and thus aninclusion of dust should be prevented during a pelletizing step, or aproduction or inclusion of carbonized material should be avoided duringan extruding step. Reference is made to Japanese Unexamined Pat.Publications (KOKAI) No. 58-126119 and No. 61-95914.

The above Japanese Unexamined Pat. Publication (KOKAI) No. 58-126119discloses that the optical disk substrate is molded from a pelletprepared by incorporating an antioxidizing agent into a polycarbonateflake, and pelletizing the mixture, but, the process disclosed in thepublication has disadvantages such that the melting by heating duringthe pelletizing step causes a thermal deterioration of the material, andthe inclusion of foreign substances cannot be sufficiently avoidedduring the pelletizing step.

The above Japanese Unexamined Pat. Publication (KOKAI) No. 61-95914discloses that the optical disk substrate is manufactured by carryingout a suspension polymerization of methyl methacrylate (MMA), to producea transparent acrylic resin beads, and then carrying out an injectionmolding of the beads without pelletizing. The process disclosed in thePublication, however, is applicable only to an acrylic resin having alow heat resistance, moisture resistance, and strength, and is notapplicable to a polycarbonate having excellent properties as thematerial for an optical disk, because, in the case of the polycarbonate,the polymer beads cannot be produced due to a difference in thepolymerization step.

SUMMARY OF THE INVENTION

The inventors of the present invention found that the optical disksubstrate can be evaluated quantitatively and practically by employing anew concept of a foreign-substances index.

Accordingly, the object of the present invention is to provide anoptical disk substrate, wherein a lowering of the recording and/orreading out characteristics caused by an inclusion of foreign substancesis prevented by bringing the foreign-substances index below a certainlevel.

Another object of the present invention is to provide aninformation-storage medium, and maintaining a high quality and a highreliability thereof.

Other objects and advantages of the present invention will be apparentfrom the following description.

In accordance with the present invention, there is provided an opticaldisk substrate having a foreign-substances index of not more than 1 ×10⁵μm² /g.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors employed a concept of a foreign-substances substancesindex to evaluate an amount of foreign substances contained in theoptical disk substrate. After examining a relationship between theforeign-substances index and the recording and/or reading-outcharacteristics, the inventors found that a correlation existed betweenthe foreign-substances index and an error ratio of the recording and/orreading out characteristics. Namely, the inventors found that, as acriterion for evaluating the properties of the information-storagemedium, the use of only a number of foreign substances is not accurate,but a foreign-substances index produced after due consideration of thenumber and a particle size distribution thereof is optimal.

The term "foreign-substances index" used herein means a sum of a productof (i) a square of a particle diameter and (ii) the number, of eachforeign substance (having a particle diameter of 0.5 μm or more) perunit weight. The foreign-substances are detected in a solution preparedby dissolving in an excess amount of an organic solvent (particularly,methylene chloride) a material (e.g., a raw material or a substrate) tobe evaluated. The index is calculated from the equation,

    I =Σ{[1/2(di +di)]hu 2 ×(nu -ni')]/W,

wherein I denotes the foreign-substance index, di denotes an i-thnumerical valuem (μm) for dividing a range of the particle diameter, andni denotes the number of foreign substances having a particle diameterof less than di+1 and not less than di, and detected in the solvent, ni'denotes the number of foreign substances involved in the solvent beforeuse, and w denotes the weight (g) of a material. An example of thenumerical values for dividing the range of the particle diameter is asfollows:

    ______________________________________                                        d.sub.1 =  0.5 μm    d.sub.2 =                                                                             0.6 μm                                     d.sub.3 =  0.7 μm    d.sub.4 =                                                                             1.1 μm                                     d.sub.5 =  2.5 μm    d.sub.6 =                                                                             5.0 μm                                     d.sub.7 = 10.0 μm    d.sub.8 =                                                                            20.0 μm                                     d.sub.9 = 25.0 μm.                                                         ______________________________________                                    

If foreign substances having a diameter of more than 25.0 μm aredetected, d10, d11, and so on having an appropriate numerical value areused.

The term "foreign substances" used herein means essentiallycontaminants, such as impurities, dust or carbonized material of a rawresin, which have been included in the optical disk substrate at variousstages. Nevertheless, the bit error ratio of the optical disk substratecan be accurately evaluated by using the foreign-substances indexcalculated from the "foreign substances" which are an insoluble residuein methylene chloride.

According to the present invention, the foreign-substances index of theoptical disk substrate must be 1 ×10⁵ μm² g or less, to ensure asufficient reliability as an optical substrate of an information-storagemedium. If the foreign-substances index of the optical disk substrate ishigher than 1 ×10⁵ μm2/g, the foreign substances undoubtedly cause biterrors in the information-storage medium containing such a substrate,and the reliability of the optical disk substrate will be lowered.

The foreign-substances index is preferably 5 ×10⁴ μm2/g or less, toensure greater reliability, more preferably 3 ×10⁴ μm2/g or less, toobtain an absolute reliability.

Any resin having good optical properties (such as transparency) ormoldability, may be used as a raw material for the optical disksubstrate. For example, polycarbonate, acrylic, non.crystallizablepolyolefin resin or the like may be used. Preferably, polycarbonate isused, because of a heat resistance, a mechanical strength, and a cost ofthe material.

The polycarbonate resin which may be used is not limited; for example, apolycarbonate resin having a viscosity-average molecular weight of 10000to 22000, preferably 12000 to 20000, and prepared by reacting a dihydricphenol and a carbonate, such as phosgene or diphenyl carbonate, may beused. As dihydric phenol, there may be mentioned hydroquinone,4,4'-dioxyphenyl, bis(hydroxyphenyl)alkane, bis(hydroxyphenyl)ether,bis(hydroxyphenyl)ketone, bis(hydroxyphenyl)sulfide, orbis(hydroxyphenyl)sulfone, and the lower-alkyl or halogen. substitutedderivatives thereof. Preferably, 2,2'-bis(4-hydroxyphenyl)propane(hereinafter referred to as bisphenol A),1,1-bis(4-hydroxyphenyl)ethane,2,2-bis(4-hydroxyphenyl)-hexafluoropropane or the like is used. Theabove dihydric phenol may be used alone, or in a combination thereof.

A polycarbonate partially having branched chains may be used in thepresent invention.

The viscosity-average molecular weight of 10000 to 22000 can becontrolled by adding an end-capping agent, such as p-t-butylphenol,during a step of manufacturing the polycarbonate. If theviscosity-average molecular weight is less than 10000, the strength ofthe optical disk substrate becomes too low to withstand practical use,and if the viscosity-average molecular weight is higher than 22000, anoptical disk substrate having a good moldability and optical propertiescannot be obtained.

The viscosity-average molecular weight [Mv]can be calculated bydetermining a specific viscosity [ηsp] of a solution of polycarbonate inmethylene chloride at 20° C., and using the following equations,

    ηsp/C =[η](1 +0.28ηsp)

wherein C denotes a concentration (g/1) of polycarbonate resin, and

    [η]=1.23 ×10.sup.-5 Mv.sup.0.83.

After the polycarbonate is prepared in accordance with a conventionalknown process, preferably the solution thereof is filtered, or thegranulates thereof are washed with a poor solvent, such as acetone,while heated, to remove impurities or foreign substances such aslow-molecular weight compounds, unreacted compounds, and so on. Theamount of the foreign substances, impurities, solvents and so oncontained in the raw material for injection molding should be as low aspossible.

If necessary, the raw material may contain additives, such asantioxidizing agent, e.g., phosphorus derivatives.

As a resin other than the polycarbonate resin, there may be used acrylicresin, such as polymethyl methacrylate, or a copolymer of methylmethacrylate and another methacrylate, acrylate or styrene monomer, or anon-crystallizable polyolefin, such as a random copolymer of ethyleneand a cyclo-olefin.

As mentioned above, the bit error ratio (BER) of therecording/reading-out characteristics of the optical disk can bequantitatively determined from the magnitude of the foreign-substancesindex in the disk substrate, because the bit error ratio and theforeign-substances index are correlated.

The information-storage medium according to the present invention can beproduced by forming a conventional known information-recording layer onthe above optical disk substrate.

The information-recording layer may be formed in a thickness of,generally 300 to 1000Å, by a sputtering, deposition or the like of acombination of a transition metal, such as Fe or Co, and a rare earthelement, such as Tb or Gd, for example, Gd-Fe-Co or Tb-Fe-Co material.Further, a magneto-optical disk may be produced by forming a protectivelayer having a thickness of 2 to 10 μm, from an ultraviolet-curingresin.

Apart from the above photomagnetic type, any organic recording layer,such as phthalocyanine or tetracarbonyl cyanine, may be used as theinformation-recording layer, as long as the organic layer can berecorded with a laser beam or the like.

According to the present invention, there is also provided an injectionmolding apparatus suitable for manufacturing the present optical disksubstrate. The injection molding apparatus according to the presentinvention has a structure wherein a cylinder and a screw of aconventional injection molding apparatus are replaced by a cylinderhaving an inner wall lined with a Co-Ni-Mo-Cr alloy (e.g., an alloycomprising 5 to 80 % of Co, 5 to 80 % of Ni, 0.1 to 10 % of Mo, and 1 to20 % of Cr), and a screw having a surface thereof coated with a TiClayer and a TiN layer, respectively.

The lining layer on the inner wall of the cylinder is generally formedby means of centrifugal casting or hot isostatic pressing to a thicknessof 0.5 to 5 mm. The two-layer coating is generally carried out byforming the TiN layer (to a thickness of 1 to 20 μm) on the TiC layer(to a thickness of 1 to 20 μm), by means of chemical vapor deposition orphysical vapor deposition method.

When an injection molding is carried out with a conventionalinjection-molding apparatus, usually, some resin passing through theapparatus remains therein, or is unevenly heated. Accordingly, themelted resin is partially carbonized, to form a carbonized material,which in turn is included in the resin as a foreign substance. Further,a foreign substance which has been bonded to various members of theinjection-molding apparatus but has not been removed during a cleaningstep, is incorporated into the resin. Under these circumstances, whenthe conventional apparatus is used, the foreign-substances index of theresin is increased by 1 ×10⁴ to 5 ×10⁵ μm2/g, a step of charging the rawmaterial into a hopper to a final step of obtaining the molded opticaldisk substrate. In particular, the partial carbonization of the resinconsiderably affects the increase of the foreign-substances index.

In the injection-molding apparatus according to the present invention,the inner wall of the cylinder has a lining of the Co-Ni-Mo-Cr alloy,and because this alloy lining has a good corrosion resistance andnon-adhesiveness, the bonding or remaining of the resin is reduced,whereby the above-mentioned partial carbonization or deterioration isavoided.

Further, the surface of the screw in the apparatus of the presentinvention has a two-layer coating, i.e., the TiC layer and the TiNlayer, and due to the non-adhesiveness of the resin to the surface ofthe screw, the burning of remaining resin is prevented and theproduction of carbonized material inhibited. Further, because thetwo-layer coating has not only the TiC layer but also the TiN layer, asthe surface layer, the oxidation resistance at an elevated temperatureis enhanced, and thus, the production of carbonized material is evenmore effectively prevented.

When polycarbonate is used as the raw resin for the optical disksubstrate according to the present invention, the resin having aforeign.substances index of, usually 30000 μm2/g or less, preferably15000 μm2/g or less, is used. During the injection molding, thetemperature of the resin is 300 to 400° C., and the temperature of themold is usually 50 to 140° C.

It is to be noted that only a surface temperature of the mold is madehigher than a glass transition temperature of the resin, using a highfrequency or the like, and after injecting the resin, the resin may becooled to a temperature at which the disk substrate can be taken out.This process results in a substrate having improved optical properties.

The injection-molding apparatus according to the present invention canbe used to produce a molded article having a low concentration offoreign substances contained therein.

According to a process of the present invention for manufacturing theoptical disk substrate, a disk substrate having a foreign-substancesindex of not more than 1 ×10⁵ μm2/g can be produced by carrying out aninjection-molding of an unmelted polycarbonate powder having aviscosity-average molecular weight of 10,000 to 22,000. Preferably,powders having a particle diameter of 500 to 2000 μm account for atleast 95% by weight of the total polycarbonate powder. Further, thepolycarbonate powder preferably has a bulk density of 0.6 to 0.8 g/cm3.

When the disk substrate is produced by directly carrying out aninjection-molding of unmelted polycarbonate powder, theforeign-substances index of the raw material can be lowered, andfurther, a heat history of the resin may be limited to only oneoperation, whereby pyrolysis of the resin is reduced and the inclusionof carbonized material minimized, and therefore, a high-performanceoptical disk substrate having a low foreign-substances index andexhibiting a low bit error ratio is easily obtained.

A preferred embodiment of a process for manufacturing the optical disksubstrate according to the present invention will be describedhereinafter.

Polycarbonate resin is used as the raw material for the disk substrate.This polycarbonate resin has a viscosity.average molecular weight of10000 to 22000, preferably 12000 to 20000, and is an unmelted powder,not a melted pellet as generally used.

The viscosity-average molecular weight should be 10000 to 22000, toensure a good moldability by which enhanced and uniform opticalproperties, such as transparency or birefringence, or a higher strengthof the optical disk substrate, are obtained.

The polycarbonate resin which may be used is not limited, and any of theabove-mentioned resins may be used.

As mentioned above, after the polycarbonate is prepared in accordancewith a conventional known process, preferably the solution thereof isfiltered, or the granulates thereof are washed with a poor solvent, suchas acetone, while heated, to remove impurities or foreign substances,such as, low-molecular weight compounds, unreacted compounds and so on.The amounts of foreign substances, impurities, solvents and so oncontained in the raw material for injection molding should be as low aspossible.

If necessary, the raw material may contain additives, such as anantioxidizing agent, e.g., phosphorus derivatives.

With respect to a particle size distribution, preferably powders havinga particle diameter of 500 to 2000 μm account for at least 95% by weightof the whole polycarbonate powder. A polycarbonate having aviscosity-average molecular weight is generally in the form of finelydivided powders, and therefore, a deaeration or uniform conveyingthereof is difficult or impossible during a melting step. Therefore, theabove particle size distribution is preferable, to ensure a stableinjection molding and obtain a good disk substrate.

More preferably, powders having a particle diameter of 700 to 1400 μmaccount for at least 95% by weight of the whole polycarbonate powder. Inthis case, the powder used has the narrow particle size distribution,and thus, a screw plastication as a melted pellet during the injectionmolding can be stably carried out.

To ensure a good fusibility, deaeration, and measurability or the like,preferably the polycarbonate powder has a bulk density of 0.6 to 0.8g/cm³.

When the optical disk substrate is manufactured from the abovepolycarbonate resin, an injection-molding apparatus, including aninjection compression molding apparatus, is used. A conventionalinjection molding apparatus may be used. To reduce the production ofcarbonized material and improve the reliability of the disk substrate,however, the above-mentioned apparatus comprising the cylinder having aninner wall lined with the Co-Ni-Mo-Cr alloy, and the screw having asurface coated with the TiC layer and the TiN layer, is preferably used.

Because the Co-Ni-Mo-Cr alloy lining on the inner wall of the cylinderand the two-layer coating of the TiC layer and the TiN layer on thescrew surface provide the surfaces thereof with a good corrosionresistance, and a non-adhesiveness of the resin to the surfaces thereof,the production of carbonized material at an elevated temperature (400°C.) can be inhibited.

During the injection molding, the temperature of the resin is 300 to400° C., and the temperature of the mold is preferably 80 to 130° C.

Only a surface temperature of the mold may be higher than a glasstransition temperature of the resin, using a high frequency or the like,and after injecting the resin, the resin may be cooled to a temperatureat which the disk substrate can be taken out. This process results in asubstrate having improved optical properties.

According to the present process, an optical disk substrate having adiameter of 40 to 170 mm and a thickness of 1 to 3 mm, is manufactured.The foreign-substances index of the optical disk substrate produced bythe above process is preferably 1 ×10⁵ μm² /g or less, more preferably 5×10⁴ μm² /g or less, to ensure a reduction of the bit error ratio and abetter reliability.

To lower the foreign-substances index, preferably an injection-moldingapparatus is used wherein the inner surface of the cylinder and thesurface of the screw are treated as mentioned above, and/or apolycarbonate powder including 5% by weight or less of low-molecularweight compounds (having a molecular weight of 3000 or less), 20 ppm orless of impurities, such as unreacted bisphenol A, 20 ppm or less ofsolvents, such as methylene chloride, or the like, and prepared bysufficiently repeating washing with water, acidic or alkaline solution,or a poor solvent, such as acetone (while heated, if necessary), isemployed during the step of manufacturing the polycarbonate powder.

EXAMPLES

The present invention now will be further illustrated by, but is nomeans limited to, the following Examples.

EXAMPLE 1

The injection-molding apparatus used in this example was prepared fromthe conventional apparatus (Technoplas Co., Ltd.: SIM.4749) by liningthe inner wall of the cylinder with a Co-Ni-Mo-Cr alloy (Nippon KoshuhaCo., Ltd.: K-alloy), and applying a two-layer coating of a TiC layer (4μm) and a TiN layer (4 μm) on the surface of the screw. As the moldingmaterial, a polycarbonate pellet (diameter 3 mm and thickness 3 mm)having a viscosity-average molecular weight of 15000 was used. Theforeign-substances index of the polycarbonate was 8000 μm² /g. Anoptical disk substrate (diameter 130 mm and thickness 1.2 mm) wasprepared by molding the material at a resin temperature of 360° C. and amold temperature of 120° C.

A Tb-Fe-Co recording layer was formed on the resulting disk substrate byspattering, to obtain an information-storage medium. The bit error ratioof the medium was measured by OMS-1000 (Nakamichi Co., Ltd.).

The foreign-substances index of the disk substrate and the bit errorratio of the medium are shown in Table 1.

EXAMPLE 2 and 3

The procedure described in Example 1 was repeated, except that apolycarbonate pellet having a foreign-substances index of 3500 μm² /g(Example 2) or 2500 μm² /g (Example 3) was used. The resulting substrateand medium were evaluated as above, and the results are shown in Table1.

COMPARATIVE EXAMPLE

The procedure described in Example 1 was repeated, except that the screwsurface was coated with Ni-SiC, and the inner wall of the cylinder wassubjected to a nitriding treatment. The resulting substrate and mediumwere evaluated as above, and the results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Foreign-substances                                                                        Bit error ratio                                                   index (μm.sup.2 /g)                                                                    (Bit/Bit)                                               ______________________________________                                        Example 1   1.5 × 10.sup.4                                                                        1.8 × 10.sup.-6                               Example 2   9.5 × 10.sup.3                                                                        0.7 × 10.sup.-6                               Example 3   4.8 × 10.sup.4                                                                        2.0 × 10.sup.-5                               Comparative 1.9 × 10.sup.5                                                                        1.5 × 10.sup.-4                               Example                                                                       ______________________________________                                    

It is apparent from the Table 1 that the optical disk substratesprepared by the apparatus of the present invention have lowerforeign-substances indexes, and the storage mediums obtained therefromhave lower bit error ratios, in comparison with those prepared in theComparative Example.

EXAMPLES 4 TO 6

As a molding material, an unmelted polycarbonate (viscosity.averagemolecular weight 15600: powders having a diameter of 700 to 1400 μmaccounting for 96.5% by weight of the whole powder: bulk density 0.68g/cm³) as shown in Table 2 was used, respectively. An optical disksubstrate (diameter 130 mm and thickness 1.2 mm) was molded at a moldingtemperature of 360° C. and a mold temperature of 120° C., using aninjection-molding apparatus treated as indicated in Table 2.

As in Example 1, a Tb-Fe-Co recording layer was formed on the resultingdisk substrate by spattering, to obtain an information-storage medium.The bit error ratio of the medium was measured as in Example 1.

The foreign-substances index of the disk substrate and the bit errorratio of the medium are shown in Table 2.

EXAMPLE 7

The procedure described in Example 4 was repeated, except that a meltedpolycarbonate pellet (viscosity-average molecular weight 15400, diameter3 mm, and thickness 3 mm) was used instead of unmelted polycarbonate.The results are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________             Examples                                                                      4        5        6     7                                            __________________________________________________________________________    Raw material                                                                           unmelted unmelted unmelted                                                                            pellet                                                PC*      PC*      PC*   PC*                                          FSI** of 4800     1500     4800  15000                                        raw material                                                                  Cylinder of                                                                            Co--Ni--Mo--Cr                                                                         Co--Ni--Mo--Cr                                                                         nitride                                                                             Co--Ni--Mo--Cr                               molding machine                                                                        coating  coating        coating                                      screw of TiC/TiN  TiC/TiN  Ni--SiC                                                                             TiC/TiN                                      molding machine                                                                        coating  coating  plating                                                                             coating                                      FSI** of 8.5 × 10.sup.3                                                                   5 × 10.sup.3                                                                     2.5 × 10.sup.4                                                                3.3 × 10.sup.4                         substrate                                                                     BER***   1.1 × 10.sup.-6                                                                  5 × 10.sup.-7                                                                    4.5 × 10.sup.-6                                                               1.3 × 10.sup.-5                        (Bit/Bit)                                                                     __________________________________________________________________________     PC* . . . polycarbonate; FSI** . . . foreignsubstances index; BER*** . .      bit error ratio.                                                         

It is apparent from the Table 2 that the optical disk substratesprepared by using an unmelted polycarbonate have improvedforeign-substances indexes, and the storage mediums obtained therefromhave improved bit error ratios, and the unmelted polycarbonate providesa high performance disk substrate.

Although the present invention has been described with reference tospecific embodiments, various changes and modifications obvious to thoseskilled in the art are deemed to be within the spirit, scope and conceptof the invention.

We claim:
 1. An optical disk substrate having a foreign-substances indexof not more than 1 ×10⁵ μm² /g; said optical disk substrate beingprepared from polycarbonate having a viscosity average molecular weightof 10,000 to 22,000.
 2. An optical information-storage medium,comprising an optical disk substrate having a foreign-substances indexof not more than 1 ×10⁵ μm² /g and a layer formed thereon for recordinginformation; said optical disk substrate being prepared frompolycarbonate having a viscosity average molecular weight of 10,000 to22,000.
 3. An optical disk substrate according to claim 1, wherein saidforeign-substances index is determined by the number of the particles ofthe foreign substances and particle diameter distribution of the foreignsubstances detected in the optical disk substrate.
 4. An opticalinformation-storage medium according to claim 2, wherein saidforeign-substances index is determined by the number of particles of theforeign substances and particle diameter distribution of the foreignsubstances detected in the optical disk substrate.
 5. An optical disksubstrate prepared from polycarbonate, said polycarbonate having aviscosity average molecular weight of 10,000 to 22,000, and having aforeign-substances index of not more than 1 ×10⁵ μm² /g, which iscalculated from the equation:

    I=Σ1/2)d.sub.i +d.sub.i+1).sup.2 ×(n.sub.i -n.sub.i) ÷W

wherein I denotes the foreign-substances index, di denotes an i-thnumerical value (μm) for dividing a range of the particle diameter, then_(i) denotes the number of foreign substances having a particlediameter of less than d_(i) +1 and not less than d_(i), and detected ina solvent, n_(i) denotes the number of foreign substances involved inthe solvent before use, and W denotes the weight (g) of thepolycarbonate.
 6. An optical information-storage medium, comprising anoptical disk substrate prepared from polycarbonate, said polycarbonatehaving a viscosity average molecular weight of 10,000 to 22,000, andhaving a foreign-substances index of not more than 1 ×10⁵ μm² /g, whichis calculated from the equation:

    I=Σ1/2(d.sub.i +d.sub.i+1)hu 2 ×(n.sub.i -n i')}÷W

wherein I denotes the foreign-substances index, di denotes an i-thnumerical value (μm) for dividing a range of the particle diameter, andn_(i) denotes the number of foreign substances having a particlediameter of less than d_(i+1) and not less than d_(i), and detected inthe solvent, n_(i) denotes the number of foreign substances involved inthe solvent before use, and W denotes the weight (g) of thepolycarbonate.